Functional dynamics in framework materials

Dynamic crystalline materials have emerged as a unique category of condensed phase matter that combines crystalline lattice with components that display dynamic behavior in the solid state. This has involved a range of materials incorporating dynamic functional units in the form of stimuli-responsive molecular switches and machines, among others. In particular, it has been possible by relying on framework materials, such as porous molecular frameworks and other hybrid organic-inorganic systems that demonstrated potential for serving as scaffolds for dynamic molecular functions. As functional dynamics increase the level of complexity, the associated phenomena are often overlooked and need to be explored. In this perspective, we discuss a selection of recent developments of dynamic solid-state materials across material classes, outlining opportunities and fundamental and methodological challenges for their advancement toward innovative functionality and applications.

(3) The use of the term "omnidirectional" seems inappropriate. Omnidirectionality implies that a given molecular entity can explore all the degrees of freedom that it has in the gas phase, or in a noninteracting liquid. The fact is that this is never the case when constrained environments reach the nanometer scale. Furthermore, the example shown with a floppy side chain linked to the framework cannot be omnidirectional as shown by the fact that it they are attached by a fixed permanently directional bond and cannot experience all rotational degrees of freedom. There must be a better term to describe this case. Constrained dynamics? Disordered dynamics?... (4) Figure 2 is not useful if it is not more explicit as to what it all means, with the trajectories and dynamics of the corresponding elements. It is not clear what it means for a dynamic framework to be multidimensional. The message in Figure 3 is also poorly represented.
(5) In page 3 second paragraph the authors refer to "strong" intermolecular pi-pi interactions as resulting in large barriers, which are not present in framework materials. In fact, pi-pi are not considered strong. Subsequently the authors offer a single example to make the comparison between molecular crystals and frameworks and they select a non-aromatic structure, bicyclo[2.2.2]octane. However, browsing through the literature one can find examples of molecular crystals with aromatic phenylenes rotators experiencing barriers lower than those of methyl groups in the gas phase, and one can also find open frameworks with rotators that have to surmount a very large intrinsic barriers, such as the iconic MOF 5. A recent JOC review article explicitly covers variations between different types of solids based on intrinsic barriers, free volume, and correlated dynamics that provides a general framework for analysis. (6) In the last paragraph in Figure 4 the authors give a list of stimuli reported to influence the dynamics or properties of framework materials. It would be very useful for the interested readers to have references to those reports. (7) The section on the assessment of functional materials across length scales in page 4 describes some of the analytical tools to measure dynamics in extended frameworks and other solids. It seems that this section is missing some interesting tools. This include the analysis of anisotropic displacement parameter (ADP) available from variable temperature single crystal x-ray diffraction, the informationa available from crystallographic disordered, which can be static or dynamic, and the use of frequency and temperature-dependent dielectric spectroscopy, and even inelastic neutron scattering for ultrafast dynamics. Giving the reader and idea of what method to use in what dynamic range would be greatly appreciated.
(8) The paragraph on dynamic cooperativity in dynamic crystals would not be complete without considering the 2020 Chemical Sciences assay on correlated motion and mechanical gearing by the Garcia-Garibay group.
To conclude, this reviewer feel that this perspective has great potential, but it needs a little work with the goal of becoming more scholarly, educational, and broad reaching.
Reviewer #3 (Remarks to the Author): In the present perspectives article, some aspects of light-responsive framework materials are addressed. The focus is on one kind of molecular motors in COFs and MOFs. At first, it is not obvious why the focus was chosen, see below. I reckon that the perspective may eventually published, but I have my reservations that a journal like Communications Chemistry is appropriate. A more specific journal is more appropriate.
Before resubmission, some issues need to be revised: -The article is filled with strong catchphrases without providing scientific insights. At least some solid scientific insights need to be provided. -The title does not match the content. A more accurate title is needed.
-The addressed dynamics focus on light-driven molecular motors of special kind. By checking the cited references, it is obvious the focus is on overcrowded-alkene-molecular-motors. This needs to be stressed in the text.
- Figure 1 shows zeolites and perovskites. To term them framework materials is uncommon.
-The dynamics in zeolites and perovskite are not discussed in the text or, at least, mentioned. Especially the early work of light-switched zeolite membranes seems important to provide a time line for the field of dynamic solid materials.
-The authors should explain what they mean with the term "mechanism", used in Figure 3 and the abstract.
-The metaphors of the 4 climbers and the map in the abstract figure remain mysterious to me, even after spending some time trying to comprehend it. It should be revised.
-The several statements in the conclusion are hard to follow: "One of the limiting factors refers to the toolbox for analyzing dynamics in the solid-state, especially from the perspective of in-situ structural characterization methods in response to external stimuli." -The methods summarized in https://doi.org/10.1021/acs.chemrev.1c00528 were used… "While functional dynamics opens the way to unique phenomena, we believe that it is critically important to reach beyond conventional boundaries of research disciplines and material classes to overcome some of the pressing challenges." -Which are?
Response: We define the term "amphidynamic materials" following the original definition by Garcia-Garibay who coined it to describe "condensed phases that combine crystalline order and liquid-like dynamics, built with lattice-forming elements linked to components that can undergo fast motion" (Proc. Nat. Acad. Sci. U.S. A. 2005, 102, 10771). This category of materials emerged to encompass solid-state systems that incorporate dynamic functional units beyond conventional structural rearrangements, such as phase transitions or lattice deformations, but other functional components that feature intrinsic dynamics in the form of molecular switches and machines referred to as "robust dynamics" (Nat. Chem. 2010, 2, 439). This does not constitute an attempt to constrain the dynamics of crystalline solids in a single label and we specify this by referring primarily to the functional dynamics in molecular framework materials (Acc. Chem. Res. 2021, 54, 1288). We clarify this in the revised manuscript. Given the various existing terminologies that can be unintentionally misleading, we describe these materials with dynamic features beyond lattice vibration and deformation as dynamic crystalline materials (DCMs), while welcoming other suggestions.
"Such efforts have led to a wide range of various dynamic crystalline materials (DCMs) known by different terms. Amphidynamic materials (ADMs) 1,2 is a term coined by M. Garcia-Garibay to describe condensed phases that "combine crystalline order and liquid-like dynamics, built with lattice-forming elements linked to components that can undergo fast motion". 3,4 ADMs have emerged as a category of condensed phase matter incorporating dynamic functional units, including molecular switches and more complex molecular machinery, such as motors and shuttles. 5 As a certain degree of dynamic molecular functionality is preserved in the solid state, more generally, DCMs exhibit structural dynamics beyond conventional structural rearrangements in condensed matter, i.e. phase transitions or lattice deformations. Instead, they feature robust dynamics, a term coined by Fraser Stoddart and Omar Yaghi to describe intrinsic dynamics within a "static" framework. 6 " (Page 1) "In this perspective, we discuss dynamic crystalline materials (DCMs) from the standpoint of criteria that define them and opportunities presented to realize functional dynamics across classes of different molecular framework materials. 18 In particular, we outline unique characteristics describing functional dynamics based on representative examples and consider methodological and other challenges for analyzing dynamic functions across length scales. Given the various existing terminologies, we describe these materials with responsive dynamic features and functionalities beyond lattice vibration and deformation as DCMs. We further address the importance of interdisciplinarity in providing a fundamental understanding of the pressing challenges, hoping to inspire researchers and new perspectives for this fascinating field." (page 1)

Continuing on the definition of the field, the title: Functional Dynamics in Framework Materials is not attractive, because the expression Framework materials is not well defined in literature and sounds rather exotic. Additionally, perovskites, extensively treated be the authors, are not generally considered as Frameworks.
Response: We appreciate the reviewer's input regarding the title and we regret that they do not find it attractive. The terminology defining "framework materials" is well-defined and appropriately used in the manuscript to refer to crystalline materials defined by periodic assemblies of specific nodes and molecular linkers, which also includes hybrid organic-inorganic perovskite materials (for reference, please refer to the Acc. Chem. Res. 2021, 54, 1288), albeit rather underrepresented in this context. We further clarify this in the revised manuscript and we adjust the title to reflect more clearly that this is a perspective on the topic .
"This applies to different crystalline (molecular) framework materials defined by periodic assemblies of various nodes and (molecular) linkers that are relevant in this context (Fig. 1a), 18 such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and coordination polymers (CPs), but also hydrogen-bonded organic frameworks (HOFs), zeolites, periodic mesoporous organosilicas (PMOs), and hybrid perovskites, among others, that have been underrepresented as prospective DCMs." (page 2) 3. The concepts of unidirectional and directional should be better distinguished and discussed, becuase they are recurrent in the manuscript: the theme is hot and deserves to be inscribed within neat boundaries to avoid confusing sentences. Experimental and theoretical results on this issue must be always distinguishable.
Response: We are in agreement with the reviewer that this terminology can be misleading and we further clarify it in the manuscript. Specifically, unidirectional motion conventionally referred to continuous directional motion based on the previous reports. However, this might be misleading as it does not include all degrees of freedom, which we further clarify in the manuscript, complementing the previous examples.
"To this end, it is essential to differentiate degrees of freedom in such molecular frameworks (i.e., rotational, translational, and conformational, etc.), and distinguish between random, restricted, controlled, and directional motion (Fig. 1a). " (page 2) "continuous (oscillatory) directional (uni)directional motion is a prerequisite for a system to perform work comparable to macroscopic machines. 38 […] Exemplary of such functionality is light-driven molecular motors, especially the overcrowded alkene-based molecular motors. Light-induced isomerization followed by a thermal relaxation step allows for a ratcheting mechanism that results in unidirectional 360° rotation. 41

Reviewer #2
This is an interesting perspective covering the field of functional dynamics in Framework materials. The subject coverage sems to be good and based on interesting from a wide range of authors. One concern is that the presentation is likely to miss the intended audience, which I assume should be relatively broad, covering chemists, physicist and materials scientists not in this field, as well as advanced undergraduate and graduate students. I will enumerate my comments and suggestions as follows.
Response: We are grateful for the positive assessment of the perspective. We share the concern about the reach of the audience, which motivates this perspective in the hope to engage a broader engagement across research fields, including chemists, physicists, and material scientists from both experimental and theoretical perspectives. We also appreciate constructive critical remarks that we address directly below. Figure 1a is probably the most useful in the entire perspective, but it could be much better if it showed how those dynamic groups connect to the extended frameworks in Figure 1b.

Most figures are not as useful as they could be. The figure in the abstract does not convey a useful message. None of the figures illustrates what ADM framework materials might looks like.
Response: We appreciate the constructive remarks of the reviewer regarding the figures. Following the criticism, we revise the figures in order to better represent the concepts. This particularly refers to the former table of content figure that now better incorporates the classes of materials represented in Fig . 1. We also provide more information on how the dynamic groups connect to the extended frameworks in Fig. 1b.   Fig. 1 Response: We agree with the assessment of the reviewer that the term "omnidirectional" might be misleading, although we refer to it based on the previous literature reports. Considering the nature of the dynamics in the relevant examples, which does not reflect "omnidirectionality", we refer to this as "restricted directional" instead of "restricted omnidirectional motion". Thank you for your consideration.
" Among the most popular examples are molecular rotors predominantly based on para-substituted cyclic molecules (e.g., aromatics) with low torsional barriers featuring restricted directional motion. 13 " (page 3) "While this illustrates the power of enhancing and facilitating specific dynamics in the solid state, the underlying motion is dictated by the molecular structure of the dynamic unit and its surroundings, driven by temperature and only to some degree directional in its nature." (page 3) Figure 3 is also poorly represented.

Figure 2 is not useful if it is not more explicit as to what it all means, with the trajectories and dynamics of the corresponding elements. It is not clear what it means for a dynamic framework to be multidimensional. The message in
Response: Following the critical remarks, we have revised Figure 2 to better represent the underlying concepts. The main features of Figure 3 are now refined in Figure 2b. Thank you for your consideration.

Fig. 2.
Schematic of the multidimensionality of the energy landscapes that can describe functional dynamics across (a) different length scales and systems, from the molecular level through the unit cell to the framework level, including host-guest systems, which (b) need to be complemented by temporal descriptors. Response: We are in agreement with the reviewer that the reference to "strong intermolecular (e.g., π-π) interactions" has been misleading in this form, as referring to the interactions more comparatively, with ππ interactions provided only as an example which plays a role in mechanically interlocked molecules detailed further in the paragraph. We have thereby revised this statement to avoid confusion while also referring to a more general overview (J. Org. Chem. 2019, 84. 9835).

In page 3 second paragraph the authors refer to "strong" intermolecular pi-pi interactions as resulting in large barriers
"While intermolecular (e.g., Van der Walls, π-π) interactions drastically enhance the barrier for rotation in densely packed molecular crystals, spatial separation of adjacent rotating molecules in molecular frameworks allows unrestricted rotational dynamics within a rigid 3D lattice.

In the last paragraph in Figure 4 the authors give a list of stimuli reported to influence the dynamics or properties of framework materials. It would be very useful for the interested readers to have references to those reports.
Response: Following the reviewer's remark, we further outline the references to the paragraph (on page 5) detailing different stimuli used to control molecular dynamics in DCMs, which are further discussed below.
"While precise structural arrangements permit controlling molecular dynamics in DCMs, control over such motion is possible through external stimulation that enables functional response beyond temperaturecontrolled random motion and towards (uni)directional motion. To this end, different stimuli have been applied, 35-37,49,50-51 including chemical processes (e.g., pH changes or chemical fuels), 37,51 electrical charge, 37 light, 35,43 and mechanical pressure. 19,52 " (page 5) 7. The section on the assessment of functional materials across length scales in page 4 describes some of the analytical tools to measure dynamics in extended frameworks and other solids. It seems that this section is missing some interesting tools. This include the analysis of anisotropic displacement parameter (ADP) available from variable temperature single crystal x-ray diffraction, the informationa available from crystallographic disordered, which can be static or dynamic, and the use of frequency and temperature-dependent dielectric spectroscopy, and even inelastic neutron scattering for ultrafast dynamics. Giving the reader and idea of what method to use in what dynamic range would be greatly appreciated.
Response: We appreciate the reviewer's input and we expand the discussion on the relevant analytical tools. In addition, we add a remark on the relevance of these methods within different dynamic ranges and complement with Figure 3 as an overview of some of the methods (inspired by previous reference reports).
" This requires a combination of theoretical and experimental techniques that involve structural assessment as well as the analysis of optoelectronic characteristics of the materials via a range of spectroscopic and spectroelectrochemical techniques (Fig. 3). 61-65 In particular, structural characteristics of DCMs are commonly analyzed using X-ray diffraction techniques in conjunction with theoretical models, as well as pair distribution function (PDF) analysis. 35,48 Furthermore, the analysis of anisotropic displacement parameters (ADP) available from variable temperature single crystal X-ray diffraction is relevant, along with the use of frequency and temperature-dependent dielectric spectroscopy and inelastic neutron scattering for ultrafast dynamics. 64,65 Modern time-resoled electron diffraction experiments allow following light-induced processes on the nanoscale, drastically enhancing the time and space resolution of crystallographic methods. 66 Although these methods permit the assessment of crystallographic characteristics, establishing accurate modes in terms of extended structural complexity and dynamics levels is an ongoing challenge. This could be addressed by considering complementary techniques, such as Raman/IR and solid-state NMR spectroscopy, which remain underrepresented in this context despite the capacity to offer atomic-level insights. 20-22 " (page 7)

9.
To conclude, this reviewer feel that this perspective has great potential, but it needs a little work with the goal of becoming more scholarly, educational, and broad reaching.
Response: We thank the reviewer once more for their constructive input and hope that they find the revised manuscript more appropriate with respect to scholarly presentation and broader audience and impact.

Reviewer #3
In the present perspectives article, some aspects of light-responsive framework materials are addressed. Response: We thank the reviewer for their evaluation of the perspective article. We are in agreement with the reviewer that the research on light-responsive framework materials, particularly COFs and MOFs, is well documented in a number of recent review articles, which we also refer to in the manuscript. However, this perspective does not aim to provide a comprehensive review of the literature on this specific topic and goes beyond this research scope by addressing the question of functional solid-state dynamics across material classes, with a particular focus on framework materials. Apart from typical molecular frameworks, such as COFs and MOFs, this also involves coordination polymers, hydrogen-bonded organic frameworks, zeolites, periodic mesoporous organosilicas (PMOs), and hybrid perovskites that have been underrepresented in this context, as detailed in the manuscript. Moreover, we critically assess the criteria that define (amphi)dynamic solid-state materials and the methodology to assess them toward better fundamental understanding and practical applications. This unique perspective thereby distinguishes our work from the previous reports, which we believe makes it appropriate for the Communications Chemistry journal considering its broad research audience, and we appreciate the reviewer's consideration.
Following the critical remarks, we further clarify and emphasize the topical scope in the revised manuscript.
"In this perspective, we discuss a selection of recent developments of dynamic solid-state materials across material classes, outlining opportunities and fundamental and methodological challenges for their advancement toward innovative functionality and applications." (page 1) "The research on DCMs develops at the intersection of several disciplines […] which we do not review here in detail. 3,8-16,17 " (page 1) "In this perspective, we discuss dynamic crystalline materials (DCMs) from the standpoint of criteria that define them and opportunities presented to realize functional dynamics across classes of different molecular framework materials. 18 In particular, we outline unique characteristics describing functional dynamics based on representative examples and consider methodological and other challenges for analyzing dynamic functions across length scales. Given the various existing terminologies, we describe these materials with responsive dynamic features and functionalities beyond lattice vibration and deformation as DCMs. We further address the importance of interdisciplinarity in providing a fundamental understanding of the pressing challenges, hoping to inspire researchers and new perspectives for this fascinating field." (page 1) "This applies to different crystalline (molecular) framework materials defined by periodic assemblies of various nodes and (molecular) linkers that are relevant in this context (Fig. 1a), 18 such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and coordination polymers (CPs), but also hydrogen-bonded organic frameworks (HOFs), zeolites, periodic mesoporous organosilicas (PMOs), and hybrid perovskites, among others, that have been underrepresented as prospective ADMs." (page 2) "While functional dynamics opens the way to unique phenomena, reaching beyond the conventional boundaries of research disciplines and material classes associated with solid-state dynamics is essential to overcome some pressing challenges. 78 " (page 9) We further address the critical points directly below.

5.
The dynamics in zeolites and perovskite are not discussed in the text or, at least, mentioned. Especially the early work of light-switched zeolite membranes seems important to provide a time line for the field of dynamic solid materials.
Response: In addition to the examples of hybrid perovskites that were previously provided in the perspective, following the reviewer's remark, we also refer to the examples of light-switched zeolite membranes. We thank the reviewer for the insightful remark.
"Such functional dynamics based on random molecular motion is relevant for other hybrid organicinorganic framework materials, such as hybrid metal halide perovskites. [18][19][20][21][22] They have demonstrated a unique capability to incorporate various organic species within their crystalline lattice that is otherwise primarily defined by the inorganic metal-halide framework. 18-22 As a result, the overall structural properties and their functional dynamics could be controlled by an interplay of interactions between the organic and inorganic components in response to various external stimuli. 23 For instance, this enables reversible pressure-induced mechanochromism in these materials and, since recently, the integration of stimuli-responsive molecular components within the perovskite scaffold, opening a path towards multifunctional materials. 17-18,23 " (page 3) "Similarly, cations in aluminium-rich zeolites exhibit dynamics within the cavities, blocking the transport of guest species. 24 In addition, light-controlled zeolite membranes have shown dynamic solid-state functionality. For instance, azobenzene incorporation into the zeolitic cavities was used to tailor its permeation, 25 illustrating the capacity of other various frameworks in realizing functional dynamics within pore cavities." (page 3)